1. Field of the Invention
The subject of the invention is a novel method for preparing a thin ceramic and/or metallic solid-state composition consisting of three phases: a material (A), a material (B) and pores. The concentration of each phase varies continuously from one face of the article to the other in a continuous and controlled gradient. The porous matrix of material (A) has a porosity gradient of 0% to about 80%, the pores being completely or partly filled with material (B). The concentration of material (B) in the article therefore varies from 80% to 0% of small thicknesses.
2. Related Art
Porous ceramics have physico-chemical properties, whether thermal stability, chemical stability, biocompatability or mechanical strength, which make them good candidates for various applications such as filter membranes, sensors, ceramic-to-metal seals, biomaterials, energy conservation, thermal insulation or catalysis. These materials are used in particular for their low density, their high exchange area and their high permeability thanks to their open porosity.
As techniques for creating porosity in a ceramic, there are:                incomplete sintering of ceramic particles;        the introduction of porosity by emulsion of the material before sintering;        the use of pore formers removed before sintering;        forming operations such as extrusion, injection molding, rapid prototyping; and        the use of ceramic fibers.        
These methods are listed in Roy W. Rice, “Porosity of ceramics”, Marcel Dekker, 1998, pp 20-21.
Incomplete sintering or subsintering of a ceramic powder or of a blend of ceramic powders having different particle sizes does not allow a porosity of greater than 50% to be achieved.
The use of pore formers, removed for example by pyrolysis before sintering, and leaving pores as the negative thereof in the ceramic, is one of the most appropriate methods for producing materials whose porosity is controlled in terms of volume fraction, shape and size distribution of the pores. Incorporating particulate pore formers, such as starch, lattices, graphite or resins into ceramic suspensions or slurries makes it possible to obtain uniformly distributed pores in a dense ceramic matrix. Depending on the forming method—pressing, casting in a mold, tape casting, extrusion or injection molding—a material is obtained with a plane geometry, a tubular geometry or a geometry of more complex shape.
Several embodiments of this technique of incorporating pore-forming particles into a ceramic suspension are disclosed in United States patents published under the numbers U.S. Pat. No. 4,777,153, U.S. Pat. No. 4,883,497, U.S. Pat. No. 5,762,737, U.S. Pat. No. 5,846,664 and U.S. Pat. No. 5,902,429 and in the publications by Lyckfeldt et al. and Apté et al. (O. Lyckfeldt, E. Lidén, R. Carlsson, “Processing of thermal insulation materials with controlled porosity”, Low expansion materials, pp 217-229; S. F. Corbin, P. S. Apté, J. Am. Ceram. Soc., 82, 7, 1999, pp 1693-1701). Apté et al. describe in particular a method using the tape casting of ceramic suspensions containing pore-forming particles and the thermocompression of tapes in order to obtain, after sintering, a porous material with a discrete porosity gradient.
The pore former may also be a preform impregnated with a ceramic suspension (ceramic powder+solvent+organic additives).
The infiltration of polymer foams by a ceramic suspension is used to obtain bulk ceramics having a substantial open porosity. In this case, the total porosity is directly due to the structure of the foam, but this technique does not allow micron pore sizes to be achieved and cannot be used to prepare thin materials.
U.S. Pat. No. 4,780,437 discloses a method for preparing thin porous materials by infiltration of a flocking of pyrolyzable pore-forming fibers by a ceramic suspension. The materials obtained by this method have oriented anisotropic pores.
Controlling the structure, whether as a dense system or a porous system with a porosity gradient, and controlling the microstructure, especially the particle size distribution and the pore size distribution of a ceramic article, is a key factor as regards its intrinsic properties and as regards its applications in terms of performance, reproducibility, lifetime and cost.
At the present time, it is not known how to manufacture a thin ceramic membrane, having a thickness of a few hundred microns, possessing a continuous controlled surface porosity gradient ranging from 0% (dense ceramic) to about 80% (highly porous system) in a single operation. All the articles produced using the various known methods have discrete or discontinuous controlled porosity gradients. Now, the presence, even in the same material, of these discrete porosity gradients may cause, at the various interfaces, layer debonding and delamination phenomena, especially because of the differences in thermal expansion coefficients between these regions. This results in rapid degradation of the article.
The fact of being able to produce a continuous controlled surface porosity gradient in a material should prevent the succession of interfaces between the layers of different porosity and consequently avoid these degradation phenomena.
In the production of electrochemical cells formed from a dense solid-state electrolyte and electrodes, called volume electrodes, such as those described in international patent application WO 95/32050, the fact of controlling a microstructure of the solid-state electrolyte with a continuous controlled surface porosity gradient completely or partly filled with an electrode material should make it possible:                to promote physical compatibility and chemical compatibility between volume electrode and dense solid-state electrolyte and thus improve the cohesion of the interface between these two materials;        to limit the energy costs associated with interfacial overpotentials; and        to promote the diffusion, disassociation and recombination of oxygen throughout the three-dimensional edifice of the volume electrode/dense solid-state electrolyte porous structure, by uniformly delocalizing volumewise the electrode reaction.        
The electrochemical cells thus formed have improved performance in terms of electrochemical performance (current density applied per unit area), lifetime, aging (degradation) and energy cost.
In the case of the production of solid-state fuel cells or SOFCs (solid oxide fuel cells), these are formed from a dense solid-state electrolyte of small thickness (between 5 μm and 300 μm, preferably between 10 μm and 100 μm) deposited either on the anode electrode (fuel side) or on the cathode electrode (air side). The fact of controlling a solid-state electrolyte structure/microstructure (thickness, density, particle size, porosity with a continuous compositional gradient created by total or partial filling of a continuous controlled surface porosity gradient of one of the “support” electrode (anode or cathode) materials should make it possible:                to promote physical compatibility and chemical compatibility between said anode (fuel) or cathode (air) “support” electrode and the dense solid-state electrolyte and thus improve the cohesion of the interface between these two materials;        to limit the energy costs associated with interfacial overpotentials and with the thickness of the solid-state electrolyte; and        to promote the diffusion, dissociation and recombination of oxygen throughout the three-dimensional edifice of the “anode or cathode” volume electrode/dense solid-state electrolyte porous structure by uniformly delocalizing volumewise the electrode reaction.        
The solid-state fuel cell elements thus formed have improved performance in terms of productivity (higher power produced per unit area), lowered operating temperature, lifetime, aging (degradation) and energy cost.
In the case of the production of a catalytic membrane ceramic reactor for the reaction for example, of reforming methane into a syngas according to the chemical reaction CH4+½O2→2H2+CO, the dense membrane is a material having a crystal structure of the ABO3, AA′BB′O6 (A,A′: lanthanide and/or actinide; B,B′: transition metal), brown-millerite and/or pyrochlore perovskite type. The material possesses mixed conductivity properties and is deposited in the form of a dense membrane (density>94%) with a thickness of between 5 μm and 500 μm, preferably between 10 μm and 300 μm, on a porous support of the same chemical composition or of a different chemical composition. The fact of controlling a structure/microstructure (thickness, density, particle size, residual porosity) of the dense (mixed conducting) membrane with a continuous compositional gradient by total or partial filling of a continuous controlled surface porosity gradient of the support should make it possible:                to promote physical compatibility and chemical compatibility between said porous support and the dense mixed conducting membrane and thus improve the cohesion of the interface between these two materials;        to increase the flux of oxygen produced, this flux being, according to Wagner's law, inversely proportional, for a given working range, to the thickness of the dense membrane;        to promote the diffusion, dissociation and recombination of oxygen throughout the three-dimensional edifice of the porous structure of the support if the latter is of the same chemical composition as the membrane; and        to improve the mechanical integrity of the membrane reactor essentially by virtue of the mechanical properties of the support.        
The mixed conducting ceramic membrane for the methane reforming reaction should show improved performance in terms of oxygen flux per unit area, lowering of the working temperature, lifetime, aging (degradation) and mechanical integrity in a reducing medium compared with a self-supported system.
In this application—reforming—a catalyst is deposited on the surface of a thin dense membrane that may or may not have a developed surface and surface roughness, said membrane being supported on a porous support of the same nature or of a different chemical nature.